Healing of dermal wounds progresses through a number of phases that optimally require different wound environments of oxygen, carbon dioxide and moisture. For all but the least severe wounds, during the initial healing phase, there is a high exudation. Optimally high exudate removal and high oxygen supply are desirable during this healing phase, during which time angiogenesis and internal tissue and skin growth begin. Thereafter exudation decreases and healing progresses, with reepithelization and wound remottling beginning. During this healing phase, still high or even higher oxygen level then during the initial phase is optimal. Less exudate removal is desired to avoid desiccation of the wound. Thereafter, in the final phase of healing, reepithelization is completed, and wound approximation and closure takes place. Exudation decreases and then stops, and the final scar tissue forms, completing the healing process. During this healing phase a higher oxygen, low to dry moisture microenvironment is appropriate.
Since the wound healing phases have different optimal healing microenvironments, optimally different types of dressings should be used during the individual healing phases. These dressings should have different permeability characteristics designed to provide the appropriate gas and moisture microenvironments for each phase. In practice, different types of wound dressings, which provide appropriate microenvironments during the individual healing phases, are seldom used. The number of different types of wound dressings required is too great for practical inventorying.
Not only is there a need for a single wound dressing that can provide appropriate microenvironments during more than a single phase of wound healing, but there also is a need for a single dressing that can provide the appropriate wound microenvironment for different types of wounds. For example, it would be useful to have a single dressing that can be used for both high and low exudative wounds.
Many wound dressings in common use are occlusive, permitting little or no water vapor or gas transmission. Many others do permit water vapor and gas transmission, but are not capable of providing more than a single wound microenvironment. See U.S. Pat. Nos. 4,561,435; 4,753,231; 4,798,201; 4,813,942; 4,906,240; and EPs. 0106439; 0106440; 0174803 and 0410009.
U.S. Pat. No. 4,875,473 describes a dressing that is capable of modifying the wound environment. However, the dressing has a substantially occlusive outer cover sheet impermeable to oxygen during its initial usage. Thereafter it is converted, by removal of the impermeable outer sheet, to an oxygen permeable dressing, giving only a single aerobic wound environment phase. No concept of varying oxygen levels is disclosed.
EP 0099758 discloses a dressing capable of low initial moisture removal, convertible to high moisture vapor removal. This reference contains no disclosure concerning oxygen level control during the healing phase.